transform texture coords within OSL?
From the osl language specification under Global Variables -


  point P - Position of the point you are shading. In a displacement shader,
  changing this variable displaces the surface.
  
  float u, v - The 2D parametric coordinates of P (on the particular geometric
  primitive you are shading).


While you don't technically alter these values, you start with them and calculate an offset position that is then used to choose the output.